This application is a 371 of PCT/EP97/02497 filed May 2, 1997.
Glycolysis has been one of the first metabolic processes described in biochemical detail in the literature. Although the general flow of carbohydrates in organisms is known and although all enzymes of the glycolytic pathway(s) are elucidated, the signal which determines the induction of metabolism by stimulating glycolysis has not been unravelled. Several hypotheses, especially based on the situation in yeast have been put forward, but none has been proven beyond doubt.
Influence on the direction of the carbohydrate partitioning does not only influence directly the cellular processes of glycolysis and carbohydrate storage, but it can also be used to influence secondary or derived processes such as cell division, biomass generation and accumulation of storage compounds, thereby determining growth and productivity.
Especially in plants, often the properties of a tissue are directly influenced by the presence of carbohydrates, and the steering of carbohydrate partitioning can give substantial differences.
The growth, development and yield of plants depends on the energy which such plants can derive from CO2-fixation during photosynthesis.
Photosynthesis primarily takes place in leaves and to a lesser extent in the stem, while other plant organs such as roots, seeds or tubers do not essentially contribute to the photoassimilation process. These tissues are completely dependent on photosynthetically active organs for their growth and nutrition. This then means that there is a flux of products derived from photosynthesis (collectively called xe2x80x9cphotosynthatexe2x80x9d) to photosynthetically inactive parts of the plants.
The photosynthetically active parts are denominated as xe2x80x9csourcesxe2x80x9d and they are defined as net exporters of photosynthate. The photosynthetically inactive parts are denominated as xe2x80x9csinksxe2x80x9d and they are defined as net importers of photosynthate.
It is assumed that both the efficiency of photosynthesis, as well as the carbohydrate partitioning in a plant are essential. Newly developing tissues like young leaves or other parts like root and seed are completely dependent on photosynthesis in the sources. The possibility of influencing the carbohydrate partitioning would have great impact on the phenotype of a plant, e.g. its height, the internodium distance, the size and form of a leaf and the size and structure of the root system.
Furthermore, the distribution of the photoassimilation products is of great importance for the yield of plant biomass and products. An example is the development in wheat over the last century. Its photosynthetic capacity has not changed considerably but the yield of wheat grain has increased substantially, i.e. the harvest index (ratio harvestable biomass/total biomass) has increased. The underlying reason is that the sink-to-source ratio was changed by conventional breeding, such that the harvestable sinks, i.e. seeds, portion increased. However, the mechanism which regulates the distribution of assimilation products and consequently the formation of sinks and sources is yet unknown. The mechanism is believed to be located somewhere in the carbohydrate metabolic pathways and their regulation. In the recent research it has become apparent that hexokinases may play a major role in metabolite signalling and control of metabolic flow. A number of mechanisms for the regulation of the hexokinase activity have been postulated (Graham et al. (1994), The Plant Cell 6: 761; Jang and Sheen (1994), The Plant Cell 6, 1665; Rose et al. Eur. J. Biochem. 199, 511-518, 1991; Blazquez et al. (1993), FEBS 329, 51; Koch, Annu. Rev. Plant Physiol. Plant. Mol. Biol. (1996) 47, 509; Jang et al. (1997), The Plant Cell 9, 5. One of these theories of hexokinase regulation, postulated in yeast mentions trehalose and its related monosaccharides (Thevelein and Hohmann (1995), TIBS 20, 3). However, it is hard to see that this would be an universal mechanism, as trehalose synthesis is believed to be restricted to certain species.
Thus, there still remains a need for the elucidation of the signal which can direct the modification of the development and/or composition of cells, tissue and organs in vivo.
It has now been found that modification of the development and/or composition of cells, tissue and organs in vivo is possible by introducing the enzyme trehalose-6-phosphate synthase (TPS) and/or trehalose-6-phosphatase phosphate (TPP) thereby inducing a change in metabolic pathways of the saccharide trehalose-6-phosphate (T-6-P) resulting in an alteration of the intracellular availability of T-6-P. Introduction of TPS thereby inducing an increase in the intracellular concentration of T-6-P causes inhibition of carbon flow in the glycolytic direction, stimulation of the photosynthesis, inhibition of growth, stimulation of sink-related activity and an increase in storage of resources. Introduction of TPP thereby introducing a decrease in the intracellular concentration of T-6-P causes stimulation of carbon flow in the glycolytic direction, increase in biomass and a decrease in photosynthetic activity.
The levels of T-6-P may be influenced by genetic engineering of an organism with gene constructs able to influence the level of T-6-P or by exogenously (orally, topically, parenterally etc.) supplying compounds able to influence these levels.
The gene constructs that can be used in this invention are constructs harbouring the gene for trehalose phosphate synthase (TPS) the enzyme that is able to catalyze the reaction from glucose-6-phosphate and UDP-glucose to T-6-P. On the other side a construct coding for the enzyme trehalose-phosphate phosphatase (TPP) which catalyzes the reaction from T-6-P to trehalose will, upon expression, give a decrease of the amount of T-6-P.
Alternatively, gene constructs harbouring antisense TPS or TPP can be used to regulate the intracellular availability of T-6-P.
Furthermore, it was recently reported that an intracellular phospho-alpha-(1,1)-glucosidase, TreA, from Bacillus subtilis was able to hydrolyse T-6-P into glucose and glucose-6-phosphate (Schxc3x6ck et al., Gene, 170, 77-80, 1996). A similar enzyme has already been described for E. coli (Rimmele and Boos (1996), J. Bact. 176 (18), 5654).
For overexpression heterologous or homologous gene constructs have to be used. It is believed that the endogenous T-6-P forming and/or degrading enzymes are under allosteric regulation and regulation through covalent modification. This regulation may be circumvented by using heterologous genes.
Alternatively, mutation of heterologous or homologous genes may be used to abolish regulation.
The invention also gives the ability to modify source-sink relations and resource allocation in plants. The whole carbon economy of the plant, including assimilate production in source tissues and utilization in source tissues can be modified, which may lead to increased biomass yield of harvested products. Using this approach, increased yield potential can be realized, as well as improved harvest index and product quality. These changes in source tissues can lead to changes in sink tissues by for instance increased export of photosynthase. Conversely changes in sink tissue can lead to change in source tissue.
Specific expression in a cell organelle, a tissue or other part of an organism enables the general effects that have been mentioned above to be directed to specific local applications. This specific expression can be established by placing the genes coding for TPS, TPP or the antisense genes for TPS or TPP under control of a specific promoter.
Specific expression also enables the simultaneous expression of both TPS and TPP enzymes in different tissues thereby increasing the level of T-6-P and decreasing the level of T-6-P locally.
By using specific promoters it is also possible to construct a temporal difference. For this purpose promoters can be used that are specifically active during a certain period of the organogenesis of the plant parts. In this way it is possible to first influence the amount of organs which will be developed and then enable these organs to be filled with storage material like starch, oil or proteins.
Alternatively, inducible promoters may be used to selectively switch on or off the expression of the genes of the invention. Induction can be achieved by for instance pathogens, stress, chemicals or light/dark stimuli.
Hexokinase activity is the enzymatic activity found in cells which catalyzes the reaction of hexose to hexose-6-phosphate. Hexoses include glucose, fructose, galactose or any other C6 sugar. It is acknowledged that there are many isoenzymes which all can play a part in said biochemical reaction. By catalyzing this reaction hexokinase forms a key enzyme in hexose (glucose) signalling.
Hexose signalling is the regulatory mechanism by which a cell senses the availability of hexose (glucose).
Glycolysis is the sequence of reactions that converts glucose into pyruvate with the concomitant production of ATP.
Cold sweetening is the accumulation of soluble sugars in potato tubers after harvest when stored at low temperatures.
Storage of resource material is the process in which the primary product glucose is metabolized into the molecular form which is fit for storage in the cell or in a specialized tissue. These forms can be diverse. In the plant kingdom storage mostly takes place in the form of carbohydrates and polycarbohydrates such as starch, fructan and cellulose, or as the more simple mono- and di-saccharides like fructose, sucrose and maltose; in the form of oils such as arachic or oleic oil and in the form of proteins such as cruciferin, napin and seed storage proteins in rapeseed. In animal cells also polymeric carbohydrates such as glycogen are formed, but also a large amount of energy rich carbon compounds is transferred into fat and lipids.
Biomass is the total mass of biological material.